Malaria parasites are obligate intraerythrocytic parasites that interact with their host cell in a series of membrane-associated events, viz. host cell recognition, binding to specific surface receptors, induction of erythrocyte endocytosis, interiorization of the parasite, alteration of the metabolic properties of the red cell, and lysis of the host cell with release of infective stages called merozoites. In order to gain insight into these phenomena it is proposed that the membrane proteins of the malarial parasite, both feeding (trophozoite) and infective (merozoite) stage as well as those of the infected host cell be characterized. Plasmodium lophurae, an avian malaria, grown in duckling red cells will serve as the model system for these investigations. Membrane proteins of the malaria-infected erythrocyte, and the parasite (trophozoite and merozoite stages) will be studied by polyacrylamide gel electrophoresis (PAGE) and immunoelectrophoresis. The biosynthesis of parasite membrane proteins will be investigated using radioactive amino acids, and determinations will be made whether such proteins have been inserted into the host cell plasma membrane. In addition, by the use of ferritin-labeled antibodies combined with electron microscopy it will be possible to localize the surface alterations of the erythrocyte that are parasite induced.